Beneficiation of low grade iron ores



Dec. 3, 1968 w. VOLK ET AL 3,414,402

BENEFICIATION OF LOW GRADE IRON ORES Filed June 16, 1967 LOW GRADE IRONORE l6 l4 V REDUCING 2 GAS REDUCTION GAS OUT 2o /|a 22 HEATING HEAT TO 4GAS GAS l800-2200F OUT GRIND MAGNETIC SEPARATION GANGUE HIGH PURITY IRONUnited States Patent 3,414,402 BENEFICIATION OF LOW GRADE IRON ORESWilliam Volk and Clarence A. Johnson, Princeton, N.J.,

assignors to Hydrocarbon Research, Inc., New York, N.Y., a corporationof New Jersey Continuation-impart of application Ser. No. 411,988,

Nov. 18, 1964. This application June 16, 1967, Ser.

7 Claims. (Cl. 75-26) ABSTRACT OF THE DISCLOSURE A process forbeneficiation of low grade, normally nonmagnetic, iron ores utilizingeither a fluidized bed or dilute phase reduction technique to produce amixture of reduced metallic iron and gangue, followed by a controlledheat treatment step of the mixture after which a relatively simplegrinding permits facile and high yield magnetic separation of the ironfrom the gangue.

Cross references to other applications This application is acontinuation-in-part of our application Beneficiation of Low Grade IronOres, Ser. No. 411,988, filed Nov. 18, 1964.

Background ofthe invention This invention is related to the field ofreduction and beneficiation of low grade normally non-magnetic ironores.

The beneficiation of low grade iron ores such as limonite and sideritehas been extremely difficult due to their inherent, non-magnetic qualityand the fineness of the iron particles as well as the large proportionof gangue. The iron particles are distributed throughout the gangue andeach particle is essentially encompassed by its own gangue shell. Thereduction treatment is, of course, available to the iron ore in thegangue but this only results in converting the iron from a non-magneticto a magnetic metallic iron. The problem is separating this reduced ironfrom the gangue and heretofore a satisfactory separation and recoveryhas been obtained only by a very fine and costly grinding of the reducedmixture to about 500 mesh. Normally, a grinding to this fineness isrequired to make available a suflicient amount of the iron to themagnetic separation means so as to make the process worth while. It hasbeen found, for instance, that if the grind is to only 200 meshpractically no separation can be effected. Other ores, for example,those containing iron silicates, are equally difficult to beneficiate asthe iron cannot be separated from the gangue after a simple grinding.(Mesh sizes are given according to US. Standards.)

Summary of the invention Our invention consists of an improved processfor reduction and beneficiation of low grade iron ores so that a highyield of reduced metallic iron may be obtained more economically thenheretofore has been possible.

The ore under consideration is a low grade ore normally having less than50% iron, such as limonite or siderite, which usually contain only inthe order of 30% to 40% iron. It is contacted in several stages Withhydrogen or other suitable reducing gas at a given temperature andpressure in either a fluidized dense phase bed or dilute phase typereactor so that the iron oxide component of the ore is reduced to amagnetic form, usually metallic iron. The reduced ore is then subjectedto a particular heat treatment after which it has been unexpectedlyfound, a satisfactory magnetic separation of the reduced iron in highyield can be obtained with only a 3,414,402 Patented Dec. 3, 1968 ice200 mesh grinding. It is also possible to utilize our invention with afixed bed type of reduction process. Apparently, the heat treatmentcauses a crystal growth in the iron component of the ore, or otherdisruption of the structure which results in a significant economicaladvantage of recovery of iron with our invention and a 200 mesh grindwhich is essentially the same as the usual recovery with a 500 meshgrind. The avoidance of very fine grinds is of course of great financialbenefit.

It is understood, of course, that our invention will work equally aswell with a low grade magnetic iron ore.

Our invention further consists in providing an integrated system ofpreheating, reducing, heat modifying and magnetically separating ironfrom gangue in low grade ores whereby beneficiation is renderedeconomical.

Description of the drawing The drawing is a flow diagram of the majoroperating steps for treating a low grade ore by our beneficiationprocess.

Preferred embodiment of the invention As shown in the drawing, ore froman ore supply at 10 is fed to reduction step 12. The ore may be groundprior to the reduction.

The ore is reduced substantially to metallic iron by contacting it witha reducing gas 14 such as hydrogen, carbon monoxide or combinationsthereof in the temperature range from about 850 F. to 1700 F. Normally,a reducing gas containing at least 50% hydrogen is preferred.

The contacting process may be of the fixed bed type or the fluidizedtype, either dilute or dense phase. For fluidized operation, thereducing gas is passed upwardly through the ore. Normal conditions forthe process would be a contacting temperature in the range from about850 F. to 1700 F. The gas rate would vary depending on whether theprocess was dense or dilute phase. Typical process conditions for afluidized process of the dense phase type would be a reducing gaspressure in the range of 200 to 400 p.s.i.g., a gas flow rate in therange of 0.5 to 1.0 ft./ sec. and a reduction temperature of less than1200 For this type operation, the ore would normally be ground to about20 mesh prior to pretreatment.

If a dilute phase operation were desired, the ore would normally beground to pass at least mesh and the gas flow rate would be in the rangeof 5 to 15 ft./sec. It is also possible to reduce in the dilute phase attemperatures up to 1700 F. with reducing gas pressures in the range of50 to 100 p.s.i.g. Gas is purged at 16 and may be recovered in part andreturned to the system.

This reduction step 12 may be either a single or multiple stageoperation.

After reduction, the reduced ore is passed to heating step 18. Thereduced ore may be maintained in either the dense or dilute fluidizedphase during the heating step as was done in the reduction step bypassing a heating gas upwardly through the ore to maintain fluidization.The heating gas 20 may be the same gas used in the reduction step oranother inert gas. The important characteristic of the heating gas isthat it be non-oxidizing. The gas out is shown at 22.

The temperature of the heating step is preferably in the range of 1800F. to 2200 F. The heating time is solely dependent on the type of orebeing processed and of course the extent of the conversion to metalliciron in the reduction step. However, it would normally be in the rangefrom 10 minutes to 4 hours.

If the heating step is a dilute phase operation, the metallic iron maybecome sintered, however, it is a relatively simple operation to grindthe sintered iron and gangue mixture to 200 mesh and then magneticallysepa rate the iron. It has generally been found that the heating stepcan be effectively accomplished with avoidance of sintering attemperatures in the order of 2000-2200" F. in a fluidized process if upto 1 percent by weight of carbon is added.

After the heating step 18, the reduced oregangue mixture is subjected toa mild grinding operation 24 in the order of at least passing 200 meshand this step is then followed by magnetic separation 26 with the ironat 28 separated from the gangue at 30.

The full explanation of the metallurgy of the heating step 18 in theorder of 18002200 F. is not fully known, but it is quite evident inpractice that this heating, after (or with) reduction, accomplishes ahighly desirable condition of the material. It is believed that thecrystals of iron tend to grow in size and it is apparent that the amountof iron recovered by magnetic effect materially improves.

As an example of temperature effect, the following results weredetermined on a particular ore:

ANALYSIS 100% REDUCED BASIS Temperature, Percent Percent Fe Percent F.Percent Fe gangue recovery gangue removal Ore 56. 8 43. 2 900 70. 8 29.2 89. 1 45. 8 1,400 71. 2 28.8 91. 3 40. 8 2,000 76. 9 23. 1 96. 1 60.

The best results, for this ore, were at a heating temperature of 2000"F. under which conditions 60% of the gangue could be removed by magneticbeneficiation with 96% iron recovery.

Additional illustrations of iron recoveries with various ores are shownin the following examples. In each case, the reduced ore after the heattreatment was ground to pass 200 mesh and magnetically separated fromthe gangue:

About 75% silica.

EXAMPLE III Limom'te Siderite (percent) (percent) Initial 43. 0 43. 326. 4 15. 6 Sinter Ir 83. 5 89. 6 14. 5 10. 4 86. 4 97. 5

While we have shown and described preferred forms of embodiment of ourinvention, we are aware that modifications may be made thereto and wethus desire to broadly interpret our invention within the scope andspirit of the disclosure herein and of the claims appended hereinafter.

We claim:

1. In combination with an iron ore reduction process of the type whereinthe ore is a low grade, normally nonmagnetic ore having an ironconcentration of less than 50% and the ore is reduced by contacting itat a temperature in the range from about 850 to 1700 F. with a reducinggas containing at least 50% hydrogen and the ore is then ground andmagnetically separated from the gangue; the improvement which comprises:

(a) heating the reduced ore prior to grinding to a temperature in therange from about 1800 F. to 2200 F. in the presence of a non-oxidizinggas for a time between 10 minutes to 4 hours.

2. The process as claimed in claim 1 wherein the nonoxidizing heatinggas used in the heating step is the same as the reducing gas.

3. The process as claimed in claim 1 wherein the ore is from the classconsisting of limonite and siderite.

4. The process as claimed in claim 1 wherein the reduced ore is groundto pass at least 200 mesh, after the heating step, but prior to magneticseparation.

5. The process as claimed in claim 1 wherein the ore is initially groundto pass 20 mesh, the reducing gas pressure is'in the range of 200 to 400p.s.i.g., the gas rate is within the range of 0.5 to 1.0 ft./sec. andupflow through a bed of ore and the reduction temperature does notexceed 1200 F.

6. The process as claimed in claim 1 wherein the ore is initially groundto pass mesh, the reducing gas pressure is in the range of 200 to 400p.s.i.g.; the gas rate is within the range of 5 to 15 ft./sec., and thereduction temperature does not exceed 1200" F.

7. The process as claimed in claim 1 wherein the reducing gas pressureis in the range of 50-100 p.s.i.g., the gas rate is within the range of5.0 to 15 ft./sec., the reduction temperature does not exceed 1700 F.

No references cited.

HYLAND BIZOT, Primary Examiner.

BENJAMIN HENKIN, Assistant Examiner.

